Polyimides find extensive use in electronic applications where they are useful in forming dielectric films on electrical and electronic devices such as capacitors, semiconductors and semiconductor packaging structures. Typical uses for polyimides include protective coatings for semiconductors, dielectric layers for multilayer integrated circuits, dielectric layers for multilayer integrated circuit packages, high temperature solder masks, bonding multilayer circuits, final passivating coatings on electronic devices and the like.
It is well known in the polymer art that polyimides can be made by the condensation polymerization of dianhydrides and diamines to form polyamic acid. These polyamic acids are readily dehydrated to the corresponding polyimides by heating at high temperatures, e.g. 300.degree. to 400.degree. C.
Future progress in thin film package and chip performance is largely dependent on the development of new insulating materials that can simultaneously meet a host of challenging property criteria. It is often observed that changes made to the chemical structure of a polymer that improve one property degrades another. Thus simultaneous improvement of all target properties with a given structural change is virtually unknown. One pair of properties which often display such counter trends are the dielectric constant and the glass transition temperature, two of the most important properties for a thin film insulator.
Low dielectric constant is desired for future chips and packages which will have increasingly narrower spacing between adjacent conductors. Low dielectric constant is desired to avoid cross talk and capacitive coupling between these conductors which will carry rapidly switching transient signals. High molecular weight is desirable to form a polymer film having suitable properties for a thin film insulator such as mechanical toughness which enables it to withstand the considerable stresses experienced during manufacturing of electronic components.
Diamines containing fluorine generally have a slow rate of reaction with a dianhydride and therefore will not result in a polymer having high molecular weight when processed. Polyimides are fabricated by combining the dianhydride and diamine in a solvent to intermix the diamine and dianhydride. Typically, with normal polyimides, the condensation polymerization occurs exclusively in the solvent, usually at room temperature. With some fluorinated systems of low reactivity only a limited amount of polymerization occurs in solution and an additional polymerization occurs upon drying and heating to high temperature to form a solid. The nitrogen atoms of the diamine react with the anhydride end groups of the dianhydride to form the polymer by condensation polymerization. If the amine is very reactive with the dianhydride a polymer of high molecular weight is formed. If there is a low rate of reaction between the diamine and dianhydride, an oligomer of low molecular weight is formed. Fluorine containing diamines are relatively nonreactive with dianhydrides and therefore, standard processing forms low molecular weight fluorinated polyimides, especially when the fluorinated constituents are bonded to a position ortho to the nitrogen.